The task of joining or connecting segments of pipe dominates the construction of wells and pipelines used to produce petroleum and other reservoir fluids. The cost of manufacture and quality control, and the technical attributes of the joins or connections created, provide an ever present motivation for both more efficient and versatile connections. Environmental requirements have served to heighten these demands as most of these are linear systems with little or no redundancy to mitigate the consequence of even a single connection failure.
For such tubulars, the joining methods most commonly employed are arc-welding for pipelines and threaded connections for casing or well bore completion. Both of these are ‘single point’ manufacturing processes. In arc-welding, electrode material is incrementally deposited and threaded connections are formed by the incremental removal of material during machining. Such single point processes tend to demand more detailed inspection and require greater manufacturing time than ‘global’ joining processes, such as friction welding. Furthermore, recent developments in the use of expandable tubulars to complete wellbores have placed demands on the casing more readily met by welded, rather than threaded connections.
These and other long felt industry needs have thus motivated the present inventors, to pursue discovery of ever more reliable, high quality and rapid weldments.
Similar purposes have motivated the development of other welding methods such as the “Method for Interconnecting Tubulars by Forge Welding” disclosed by Alford et al. in WO 03/055635 A1 and the related “Method for Joining Tubular Parts of Metal by Forge/Diffusion Welding” disclosed by Moe in U.S. Pat. No. 4,669,650. These methods seek to extend solid state forge and diffusion welding principles to achieve metallurgical bonding with less flash (i.e., extruded material) than typically required to forge weld and in less time than typically required to diffusion weld by the introduction of a reducing flushing fluid. The flushing fluid comprises a reducing gas such as hydrogen or carbon monoxide, perhaps mixed with a non-reactive gas such as nitrogen, and is used to blanket the mating ends of work pieces which are locally heated then pressed together to form a forge weld. The reducing gas, in combination with other means (e.g., low water vapour concentration) is understood to minimize the presence of oxides that would otherwise impede the rate and quality of bond formation. These methods, while apparently capable of rapidly producing high strength solid state welds, thus require extreme care to maintain the degree of metallurgical cleanliness required to promote bonding in both the short period of time the work piece ends remain hot (a few seconds) and without excess flash.
Diffusion bonding (interchangeably known as diffusion welding) is typically performed at stresses below that required to produce macroscopic deformation (i.e., plastic flow), and for most materials including carbon steel requires substantially oxide-free faying surfaces. The bond formation occurs over time (in the order of hours or minutes, not seconds) at elevated temperature (typically more than half the melting temperature). By contrast, conventional forge welds, such as commonly used to produce ERW (electric resistance welded) tubular products, do not require the same degree of cleanliness or time to form a bond, but these benefits come at the expense of requiring substantial plastic flow generating an upset or flash. At the forge weld temperature, typically in the recrystallization range, this plastic flow reduces bond sensitivity to the presence of oxides, because the associated large plastic deformation of the metal crystals promotes the absorption and disruption of the oxides in addition to extruding a portion into the flash, which often must be removed.
Seen in this context, it will be apparent that the weld methods taught by Alford et al, and Moe, while demonstrating that rapid ‘diffusion quality’ welds are metallurgically achievable, are relatively fragile. They are only able to enjoy the benefits of smaller flash size compared to typical forge weld requirements by providing ultra-clean faying surfaces, and they are only able to enjoy the benefit of shorter bond time (compared to typical diffusion weld requirements) by introducing significant macrosopic plastic deformation; i.e., flash size.
Another solid state welding process intended to meet this same industry need is the “rapid friction welding method” disclosed by Lingnau in patent application PCT/US99/25600, “Improved Method of Solid State Welding and Welded Parts”. This reference discloses a modified friction welding method in which the majority of the energy supplied to heat the work pieces to the hot working temperature is provided by induction pre-heating (in a non-oxidizing environment), rather than solely by kinetic energy as in conventional friction welding. This method, as described by Lingnau, enjoys several benefits over conventional friction welding, some of the primary benefits being:                large kinetic energy storage devices, such as fly wheels, or high power drives are not required, thus resulting is less bulky and costly mechanical equipment;        welds can be formed at lower surface speeds and thus rotation frequency, which is often a practical barrier, particularly in joining long tubulars; and        reduced, thinner flash results in less material waste, and in some applications flash may be small enough to leave in place, thus eliminating manufacturing steps otherwise required for flash removal.        
Even though the Lingnau method requires only a fraction of the kinetic energy of conventional friction welding (“approximately equal to 10%”, per claim 1 in Lingnau), the relative velocity between the work pieces must nonetheless be at “an initial perimeter velocity of about four feet per second [1.2 m/s]” (page 8, lines 18-19), or “the forging velocity [about 200 ft/min, or 1.026 m/s, for steel] which is much lower than the normal minimum friction welding surface velocity of 500 to 3,000 ft/min [2.54 to 15.24 m/sec] for steel” (page 10, lines 9-11). Consequently, substantial rotation or relative displacement is still expected. For example, in referring to joining 4.5 inch [114.3 mm] diameter pipe, Lingnau teaches that, “Once the hot working temperature is reached, the two work pieces are pressed together at the forging pressure, causing the rotating work piece to decelerate almost instantly, within a few revolutions.” The rotation referred to is in addition to that required prior to contact. Particularly when joining long tubular work pieces, this amount of total rotation or ‘spinning’ still introduces additional complexity and technical limitations for many applications, as for example, at pipeline tie-ins.
Similarly, while reducing the magnitude of flash generated in producing a weld compared to conventional friction welding, the method retains the concept of forging force, a uni-directional compressive force, as a necessary part of the friction weld process. It will be apparent that the hot material on the faying surfaces is thus subjected to high axial stress while simultaneously undergoing shear arising from the imposed relative transverse movement, typically imposed by rotation. This stress state necessarily results in extrusion of a significant volume of hot material, as flash, during the forge process.
In common with other friction welding, this method does not explicitly control the amount of hot working due to shear. According to accepted understandings, the large amount of shear typically imposed during friction welding of steel can sometimes lead to elongated low strength non-metallic inclusions, such as manganese sulphide (MnS), at or near the bond line. Such inclusions tend to promote crack initiation reducing weld strength, toughness and fatigue resistance.
This modified friction welding method, while avoiding the fragility of the ‘modified diffusion weld’ taught by Alford et al. and Moe, yet retains much of the operational complication or clumsiness of friction welding, associated with the need for continuous initial rotation or movement plus substantial kinetic energy input. In particular, it is advantageous if the relative movement between the work pieces can be accomplished at lower surface velocity and lower relative transverse displacement to effect a weld, even eliminating the need to “spin” altogether, while simultaneously tending to prevent excessive hot working and associated deleterious metallurgical effects, and, secondly, to further reduce the flash magnitude or eliminate it entirely.